can you do both?
Are you an aggressor or defender?
I am an aggressor. Until about the 10th move when I realize I've been too aggressive. Then I start defending until I ultimately lose.
I am usually rather attacky in the opening, but afterwards I usually always think and play defensively, until my opponent makes a great attacking blunder.
Then the back rank mate threats to my opponent start...

I am very passive in the openings, I enjoy cramped positions and prefer to let my opponent over extend themselves. Once the position is good and complicated I become much more aggressive looking to break the position open in my favor.
I suppose the best thing to do is balance each move with your mind on defense and your heart on attack.

Defender systematically builts advantages...
Systematically building advantages is not defensive, but striving to achieve dominance before launching aggression. Aggression, if premature, is usually reckless, and if defensible can lead more easily to defeat. There's a time for swashbuckling, and a time for solid, positional chess.

Both...until I get to a point where I do a better job of winning won games against very strong opposition, regardless of the margin of error. (probably Federation expert rated?). At that point, it makes more sense to have a style.
Until then, just play safe moves and hope he screws up before you do.

I hope I'm a mix of both. I think I tend a bit toward aggression because my philosophy is that aggressive play gives me some control over my opponents moves, so they become more predictable. I play for position in the opening, and then I go for aggression in midgame.
I love playing super aggressive players though because sometimes they can put up an interesting challenge, but I know that usually I will win against that kind of player.

Both...until I get to a point where I do a better job of winning won games against very strong opposition, regardless of the margin of error. (probably Federation expert rated?). At that point, it makes more sense to have a style.
Until then, just play safe moves and hope he screws up before you do.
That`s exactly where different types of people take different approach. I would "never" take such approach. I would rather MAKE him do the mistake, "make him scared." Translating into a game: trying to put him on defences by entering complications.

In chess, passive play is bad play.
The agressor usually wins in chess. It's possible to be a good counter-attacking player, but the number of truly good truly defensive players can be listed on one hand.
Ulf Andersson and Tigran Petrosian come to mind. Possibly Peter Leko? Anyone else?
"..can be listed on one hand" - that`s exactly what Kotov said in his book written in 1964 "The Art oF The Middle Game" which I read yesterday when he was talking about how many good books about middle game was written back then. What a coincidence!
Passive player in U2000 elo group will win more times a 9-round tournament than an aggressive player if that passive player does not get saduced/make a mistake by a possibility of getting a good advantage. While a passive player in above 2000 elo group may suffer too many times from players with strong openings when passive player is Black, while when White, strong Black will equalize easily. With such style above 2000 you can`t win tournaments. I am talking about real passive stuff... like Philidor and Catalan.

Let me be clear. Safe moves are "tactically safe" moves ... not passive moves ... At the club player (non-titled league), the player who wins is the one who plays tactically safer all throughout the game.
There's a difference between thinking defensively and "thinking defense FIRST".
You might ask "well of course, each move is tactically safe" but once again, consider the audience in this forum.

Let me be clear. Safe moves are "tactically safe" moves ... not passive moves ... At the club player (non-titled league), the player who wins is the one who plays tactically safe.
There's a difference between thinking defensively and "thinking defense FIRST".
I played in a U2000 elo fide 9 round tournament, the boy rated 1900 took 2nd place had 5 draws(!!!) and 4 wins. He played Phillidor set-up as Black all the time and as White he used Stonewall. That is DEFENCE FIRST!

Let me be clear. Safe moves are "tactically safe" moves ... not passive moves ... At the club player (non-titled league), the player who wins is the one who plays tactically safe.
There's a difference between thinking defensively and "thinking defense FIRST".
I played in a U2000 elo fide 9 round tournament, the boy rated 1900 took 2nd place had 5 draws(!!!) and 4 wins. He played Phillidor set-up as Black all the time and as White he used Stonewall. That is DEFENCE FIRST!
Uhh...no. Your example alludes to a person using a solid opening system.
Thinking "Defense first" is consistently looking at all your opponent's forcing responses to your planned move (before you make it) and ensuring that you can meet ALL of them, it is calculating forcing lines till quiescence and rolling up your sleeves and analyzing threats without hand waving / lazy approximations.
Experts and titled players do this a whole lot more consistently (probably a factor of 10x to 100x ) than non-titled players ... which is why until we get better with our discipline, we don't deserve a "style" ... we just need to play the "cleaner" game than our opponent and make the less costly mistakes.
I just think it's pompous to give ourselves styles when we still make basic tactical errors that a strong player wouldn't normally make.
How stupid does this sound => "Ohh, that's a ferocious 1400 player I'm playing next round, I wish I was facing the passive 1700 player...my chances are better".

If you play Phillidor kind of set-up (which can be played with both colours) you are FORCED to play passive it`s not for you to decide. Get aggressive in Phillidor and you go down.

Experts and titled players do this a whole lot more consistently (probably a factor of 10x to 100x ) than non-titled players ... which is why until we get better with our discipline, we don't deserve a "style" ... we just need to play the "cleaner" game than our opponent and make the less costly mistakes.
I just think it's pompous to give ourselves styles when we still make basic tactical errors that a strong player wouldn't normally make.
I can't argue with this on ANY level, but the way I think about the question isn't whether I choose to respond offensively or defensively, I wouldn't "say" I am either. I just think there are subconscious factors that make a person respond to threats in a specific way. When I look at my play style I don't plan aggression or passivity, I look for the best move I can see (which is why I prefer longer games) and IMO those that become great in chess must, somehow, train their minds to look past their gut reactions to a scenario, look past the obvious attacks, threats, etc. and find the right move in the right moment.
There are very aggressive Grandmaster's, but that is because they have the ability to know WHEN to be aggressive and how to take advantage of a given situation.
Just my 2 cents.

When an amateur asks this question, especially to other amateurs, maybe a little re-phraseing is in order. Some of us know how much style comes into play, or rather, doesn't come into play. I think the actual question is do you prefer playing with static or dynamic advantages?
Correctly pursuing advantageous like development and initiative often mean sacrificing and attacking in general, things an amateur is likely to associate with an "aggressive style"
While on the other hand, correctly pursuing advantages like pawn structure and space often mean maneuvering, zugzwang, and aiming to squelch the opponent's remaining play, things an amateur is likely to associate with "a defensive style"
Myself, I enjoy playing with static advantages. I'm probably a poor defender for the same reason I'm a poor attacker from a positional standpoint. The fact that I can calculate tactics that are present speaks of my calculation and pattern recognition more than any kind of style.

If you "just play safe moves and hope", you will lose against a strong player.
Again, passive chess is bad chess.
I agree passive chess is always bad chess, but there are cases where practical technique can use what's objectively bad chess to good effect. When Tal leads another GM into Salvador Dali's forest, it may be with an objectively poor sacrifice that offers good practical chances.
I think one practical advantage of playing a much weaker player is drawing the game out and waiting for a mistake. GMs are almost forced to push hard on every move, but their technique isn't comparable. Amateurs get flustered and "the need to do something" overrides their better judgement.
Anytime someone posts their amazing draw against Rybka at move 30 on a repetition or in an equal position I can't help but chuckle. Turn up contempt and play it out to an endgame and you will lose 100% of the time. Heck play nearly any equal endgame you come across against a computer from the beginning (with no take backs and such) and you'll lose the great majority of the time.

Sounds like u r saying that style does not matter to us amaturs (u including). That`s just wrong to say cos if u play sicilian and 1.e4 for attack, you attack. Cos thats ur opening choice. And if u want to defend and wait for mistakes play Phillidor set-up with both colours for example. A successfull result in a 9 round of an aggressor would be 6.5 points or more where there is max 2 draws and at least one lose. Defender will have 4 or more draws. Also defender will have much more relaxed time and use less energy. So u see there is style after all.
Defender systematically builts advantages, stays away from complications and defends from aggressor`s threats in meanwhile. Defender`s logic against Aggressor is: "Catch me if you can and if not, you will go down slowly but steadily. Well, I know draw % is high in my games but if I have to I will play more aggressive toward the end in the tournament." Aggressor`s logic: "I am the leader! I am the D O M I N A T O R! Stop me if you can. Honestly, I just find the other style lifeless."
What is your preference and why.....